This invention relates generally to X-ray detectors and more particularly concerns X-ray detectors for the control of panoramic X-ray machines.
X-ray detectors known and used in the prior art are subject to deficiences. Previous X-ray detectors have been bulky, expensive and prone to instability and parametric drift. These detectors include a fluorescent screen which upon excitation by X-rays emit light which is directed to a photo sensitive electronic tube. Electrical characteristics of the tube are such as to require a high impedance load, as well as requiring external high voltage power supplies. Photo-electric tubes are relatively bulky and are fixed in shape. The sensitive area of the tube is only a small portion of the body of the tube. Some applications require that an X-ray detector meet certain geometric configurations. The use of photosensitive vacuum tubes preclude the flexibility required by these applications.
One such application is a panoramic X-ray machine. A panoramic X-ray machine is used to take full mouth X-rays in one continuous operation. This is accomplished by providing a X-ray source on one side of the patient's skull and arranged diagonally opposite of the patient's skull, a film cassette or holder for transporting a filmstrip for recording X-ray images obtained thereby. The X-ray source and the film cassette rotate about the patient's skull in a predetermined path maintaining an area of the patient's teeth in the focal plane of interest. When the front teeth are being X-rayed, the X-ray source must be positioned behind the patient's neck while the film cassette is located in the proximity with the patient's lips. When side teeth are being X-rayed, the X-ray source on one side of the patient's face, while the cassette is on the other. While X-raying through the patient's neck, the X-ray beam must pass through the skull, vertibra and other X-ray absorbing tissue. Conversely, when X-raying teeth on the side of the patient, the X-ray encounters appreciably less attenuation. X-ray film has very limited range of exposure latitude. Thus, without compensation, a given panoramic X-ray might be overexposed as to the side teeth, but underexposed as to the front teeth. For this reason, it would be desirable for an X-ray detector to be mounted in proximity to the film cassette for providing a signal corresponding to the amount of X-rays passing through the skull. The signal may be used to amplitude modulate the X-ray source or to control the speed of rotation of the X-ray source and the film cassette. The X-ray beam passing through the skull is typically 4 or 6 inches in height and one-eighth or one-fourth inches wide at the image receptor area where detection must take place. It would, therefore, be highly desirable to provide a detector that met this geometric configuration while being devoid of external power supply or unnecessary bulk. Since devices such as the panoramic X-ray are often intended for commercial use, economy is also an important consideration.